The present invention relates to polymeric materials, a method for preparation of the polymeric materials, and the separation of mixtures using polymer membranes fabricated from the polymeric materials.
Polymer membranes have been utilized for various separations including gas separation as well as liquid separation. Membrane-based gas separation has become an important alternative to well-established separation operations, such as cryogenic distillation, and adsorption processes. Membrane-based gas separation is a pressure-driven process that does not require a high energy cost phase change of the feed gas mixture, as in other separation operations. Moreover, the mechanical simplicity and small footprint of membrane-based gas separation units provides a great deal of flexibility in installation and operation.
Such advantages have led to a wide range of applications for membrane-based gas separations. These separations include the gas pair (i.e., mixtures of at least two gases to be separated): O2/N2, H2/N2, H2/CH4, CO2/CH4, H2O/air, He/air, He/N2, He/CH4, He/H2, He/CO2, H2/CO2, H2S/natural gas and H2O/natural gas. With increasing costs of energy and environmental concerns regarding CO2 separation, collection, and sequestration, gas membrane separation offers significant promise in present and emerging industries. One emerging environmental application could involve membrane CO2/N2 separation of flue gas to allow for CO2 collection and sequestration.
The choice of a membrane material for gas separation applications is based on specific physical and chemical properties, since these materials should be tailored in an advanced way to separate particular gas mixtures. Commercial gas separation modules generally employ organic polymers as asymmetric non-porous membranes. The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by permeability and selectivity. Permeability (P) is the rate at which any gas component permeates through the membrane. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity (S) can be defined as the ratio of the permeabilities of the gas components across the membrane. The selectivity is a key parameter to achieve high product purity at high recoveries. A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Typically, polymeric membranes show relatively high selectivity and low permeability (throughput) when compared to porous materials, due to their low free volume. Polymer free volume, the fraction of the volume not occupied by the electronic clouds of the polymer, plays an important role in the transport properties of low molecular weight species and gases.
Almost all industrial gas separation membrane processes utilize glassy polymers because of relatively high gas selectivity and good mechanical properties. In glassy polymers, the more permeable species are those with low molecular diameter and selectivity is due to differences in molecular dimension. Medium to high free volume glassy polymers (e.g., polyimides, polyphenyleneoxides, poly(trimethylsilylpropyne), etc.) are used to produce membranes since the higher free volume aids the transport of gas or liquid through the material.
In addition to the overall amount of free volume, polymer membrane properties are also influenced by the size distribution and shape of free volume structure represented by micro-cavities, pores, and channels. In amorphous polymer, the size distribution and shape of free volume structures are not uniform. The broad size range and shape preclude the possibility of achieving both high selectivity and high permeability simultaneously. Thus, typical polymeric membranes generally undergo a trade-off limitation between permeability and selectivity: as selectivity increases, permeability decreases, and vice versa. Robeson showed in several references (L. M. Robeson, J. Mem. Sci. 62, 195 (1991); B. D. Freeman, Macromolecules 32, 375 (1999); L. M. Robeson, J. Mem. Sci. 320, 375 (2008)) that as for small gaseous molecules (e.g., O2, N2, CO2, and CH4) a superior limit or “upper bound” exists in a selectivity/permeability diagram. To achieve higher selectivity/permeability combinations, materials that do not obey those simple rules would be required.
A recent publication has noted that the upper bound can be exceeded with a polymer system that is thermally rearranged to promote main chain heterocyclic structures not present in the precursor polymer (Park et al., Science 318, 254 (2007)). It was noted that the pore size distribution in the thermally rearranged polymer is much narrower than in the precursor polymer, yielding molecular sieving like permeability/selectivity properties. It is believed by Park et al. that the thermal rearrangement process, not the removal of volatile gas CO2, leads to a pore size distribution narrower than the original membrane. Increasing free volume leads to increased permeability and decreasing the pore size distribution in polymers leads to increased selectivity. However, high degree of thermal rearrangement led to high crosslinking and polymer densification which in turn compromised the mechanical properties of the polymer such as tensile strength and elongation to break. Methods to achieve both high permeability and selectivity simultaneously while maintaining mechanical strength are highly desired.
Despite the foregoing developments, there is still room in the membrane separation art for further improvements.
Thus, in the design of polymeric membranes for gas separation, it is desired to increase free volume by providing pore and cavity sizes having a narrower distribution than that typically achieved with solution casting or melt processing of polymers.
It is therefore desired to provide a polymer with increased free volume and a narrow size distribution of the free volume structure, and improved mechanical properties.
It is further desired to provide a method for producing a polymer with increased free volume a narrow size distribution of the free volume structure, and improved mechanical properties.
It is still further desired to provide a gas separation membrane produced from a polymer with increased free volume and narrow size distribution of the free volume structure, and improved mechanical properties.
It is still further desired to provide a process for producing a gas separation membrane produced from a polymer with increased free volume and narrow size distribution of the free volume structure, and improved mechanical properties.
All references cited herein are incorporated herein in their entireties.